1. Field of the Invention
The present invention relates to a linear solenoid valve which includes a pressure controlling section, including a slidable valve spool biased in one direction by at least one elastic member, and an actuating section which generates a thrust force which is transmitted to the valve spool.
2. Description of the Related Art
A conventional linear solenoid valve, as shown in FIG. 6, includes a casing K of a magnetic material, a plunger (movable core) P, and a core (stationary core) F within the hollow interior portion (center space) of a core assembly C, which components form the magnetic circuit of electromagnet section M. When current is supplied to the coil C, magnetic flux is generated in the magnetic circuit, and an electromagnetic attraction force is generated between the plunger P and the core F which acts on spool valve SV of the pressure control section S which is thereby moved relative to sleeve SL to control the pressure of a fluid.
In the conventional linear solenoid valve, since a magnetic material is used for the casing K, the plunger P, and the core F, which form the magnetic circuit of the electromagnet section M, there exists a magnetic hysteresis peculiar to the magnetic material, as shown in FIG. 7. Therefore, even when the current is constant, the control pressure varies depending on the direction of movement of the spool valve, resulting in unstable operation.
Also, the amount of magnetic flux that can pass through each member made of the magnetic material is limited due to magnetic saturation and depends on the cross-sectional area of the member. Therefore, even when the current is greatly increased, the electromagnetic attraction force increases only to a limited extent, so that a large control pressure cannot be obtained.
Further, since the electromagnetic attraction force is generated in a direction along the flow of the magnetic flux, as shown in FIG. 8, a radial component of force acts on bearings B as a radial load. Therefore, the bearings B must have high accuracy, and the structure thereof becomes complicated.
Also, since sliding resistance proportional to the friction coefficient of the bearings B exists between the shaft ST and the bearings B, the above-described hysteresis increases, and the responsiveness of the control pressure to the current suffers.
Moreover, since the electromagnetic attraction force is greatly affected by the air gap between the components, the components of the electromagnet section M must be manufactured with a high degree of precision, resulting in an increase in cost.